2.1. Cells and Viruses

HEK293T cells, PK-15 cells, and BHK-21 cells were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA). Stable PK-15 cells ectopically expressing PRV UL13 (UL13-PK-15 cells), along with control cells, were generated previously [24]. Cells were maintained in Dulbecco’s minimal essential medium (DMEM) (Gibco, New York, NY, USA) containing 10% fetal bovine serum (FBS) (Biological Industries, Israel) and 1% penicillin–streptomycin (Gibco, New York, NY, USA) at 37 °C in 5% CO₂. All cells tested negative for mycoplasma using a mycoplasma detection kit (TransGen Biotech, Beijing, China). PRV (Bartha-K61 strain) was purchased from the China Veterinary Culture Collection Center (Cat# CVCC AV249, Beijing, China), and was purified in BHK-21 cells. The PRV-∆UL13 recombinant strain was generated previously [24]. The wild-type PRV (PRV-WT) and PRV-∆UL13 PRV strains were amplified and titrated in PK-15 cells using standard protocols. Sendai virus (SeV), described previously [26], was propagated in 10-day-old embryonated eggs. SeV titers were then determined via the Reed–Muench method using MDCK cells.